1. Field of the Invention
The present invention relates to a method of producing metal members which exhibit excellent oxidation resistance.
2. Description of the Related Art
Conventionally, it has been proposed to form protective coatings in surfaces of metal materials for improving the oxidation resistance thereof at elevated temperatures without lowering their mechanical characteristics and excellent properties such as good workability. As the method of forming these protective coatings, surface treatments such as plating, pack-cementation, vacuum-deposition or spraying process have been used, for example.
The surface treatment, however, has problems such as equipment being expensive, long treating time being required, intersurfaces existing between substrates and coating layers to cause peeling of the coating layers from the substrates, resulting in shortage of adhesion therebetween, and distortions and dimensional changes being generated in products due to the surface treatment.
To improve the oxidation resistance of Ti-based alloy, for example, at elevated temperatures of 600.degree. C. or more, the Ti-based alloy having a protective coating in a surface thereof has been proposed.
Japanese Patent application laid-open No. Hei 4-254597, for example, has the object of improving the adhesion and oxidation resistance, and discloses a coating composed of a ductile alloy expressed by MCrAl or MCr (Fe, Ni, Co).
Japanese Patent application laid-open No. Hei 5-345942 discloses a Ti-based alloy exhibiting oxidation resistance at elevated temperatures, of which the surface is improved by implanting ions of at least one of elements of Group Vb of the periodic table, such as phosphorus, arsenic and antimony, into an Al-containing Ti-based alloy.
Furthermore, Japanese Patent applications laid-open Nos. Hei 5-156423 and Hei 6-93412 disclose Al-Cr composite diffusion coatings, and Japanese Patent application laid-open No. Hei 9-256138 discloses a Ti-based alloy exhibiting excellent oxidation resistance and abrasion resistance, which has an Al and N-containing coating in a surface thereof.
The coating disclosed in Japanese Patent application laid-open No. Hei 4-254597 exhibits a certain degree of oxidation resistance, but does not exhibit satisfactorily good oxidation resistance. As the method of forming this coating, plasma-spraying process has been recommended, but, generally, the plasma-spraying process has defects such as (1) the treatment costs being expensive, (2) voids existing within a resultant coating to make it difficult to restrict the oxygen diffusion into a substrate, (3) adhesion between the coating and substrate being weak, and the coefficient of thermal expansion of the coating differing from that of the substrate, which result in the stability of the coating becoming worse after subjected to repeated oxidation of heating and cooling cycles.
The protective coating disclosed in Japanese Patent application laid-open No. Hei 5-345942 does not exhibit satisfactorily good oxidation resistance under severe oxidation conditions. This protective coating is formed by ion-implantation. This method, however, makes it difficult to perform the surface treatment of parts having complicated configurations.
The coatings disclosed in Japanese Patent applications laid-open Nos. Hei 5-156423 and Hei 6-93412 are formed by diffusion treatment. However, since the treating temperature ranges from 700.degree. C. to 1300.degree. C., this treatment has problems such as (1) dimensional changes of resultant parts being great, and (2) substrates being exposed to the temperature above the .alpha.-.beta. transformation temperature thereof to decrease the mechanical properties thereof.
Furthermore, with the coating disclosed in Japanese Patent application laid-open No. Hei 9-256138, the evaluation test temperature is low so that the oxidation resistance at elevated temperatures which are more severe conditions cannot be ensured. Examples of the method of forming this coating include ion-plating, sputtering, vacuum deposition, ion-implantation and CVD treatment, but, generally, these methods are expensive in treatment costs, and are not suited to the uniform surface treatment of parts having complicated configurations.
Furthermore, as the method of improving the oxidation resistance of iron-based alloy and nickel-based alloy, Japanese Patent application laid-open No. Sho 60-63364, for example, discloses the method of plating steel plates with aluminum and subjecting the aluminum-plated steel plates to the diffusion heat treatment to form coatings in surfaces thereof and improve the oxidation resistance of the steel plates.
Japanese Patent application laid-open No. Sho 60-100659 discloses that oxidation-resistant protective coatings exhibiting excellent durability are formed by plating a cast-iron member with nickel and aluminum, and subjecting the plated member to the diffusion heat treatment.
In addition to these treatments, plasma-spraying process, vacuum-deposition, ion-implantation, CVD treatment and PVD treatment are known as the method of forming the protective coatings.
The aluminum-coating treatments disclosed in Japanese patent applications laid-open Nos. Sho 60-63364 and 60-100659, however, have problems such as (1) the material coatable with aluminum being limited, (2) substrates being degraded due to the treatment at elevated temperatures, and (3) resultant coatings lacking stability over a long period due to repeated oxidation at elevated temperatures.
Furthermore, as described above, the plasma-spraying process has problems such as (1) the treatment costs being expensive, (2) voids existing within resultant coatings making it difficult to restrict the diffusion of oxygen into substrates, (3) adhesion between coatings and substrates being weak, and the coefficient of thermal expansion of the coatings differing from that of the substrates, which result in the stability of the protective coatings becoming worse against repeated oxidation of heating and cooling cycles.
In addition, ion-plating, sputtering, vacuum-deposition, ion-implantation, CVD treatment, and PVD treatment have problems such as uniform surface treatment of parts having complicated configurations being difficult, and the treatment costs being very expensive.
Furthermore, as another surface treatment of metal materials, Japanese Patent application-laid open No. Hei 10-30190 discloses a surface-improving method of metal. With the method of Japanese Patent application-laid open No. Hei 10-30190, by applying mechanical energy to surfaces of the metal in the presence of particulates of the material different from that of the metal, mechanically alloyed layers of elements composing the metal and particulates are formed.
However, with the surface treatment of metal, which is disclosed in Japanese Patent application laid-open No. Hei 10-30190, the mechanically alloyed layer formed in the surface of the metal is in a non equilibrium phase which is a metastable state like an amorphous phase or supersaturated solid solution phase so that elements contained in the metal, of which the oxidation rates are higher, are selectively oxidized with ease so that the mechanically alloyed layer has not operated sufficiently as an oxidation-resistant protective coating in an oxidation atmosphere.